by Emily Green
In high school science class, you may hear anecdotes of species that evolved into what they are today: Darwin’s finches in the Galapagos, the peppered moth during the Industrial Revolution; even our own species, Homo sapiens, evolved from an ape-like ancestor. These species all adapted to improve their survival in an ever-changing environment. However, while these examples occurred long ago, the process of evolution continues to happen all around us! Sometimes even fast enough to observe during our lifetimes. One particular species experiencing rapid evolution over the last hundred years is the Atlantic killifish. The driving force? Pollution.
The Atlantic killifish (Fundulus heteroclitis), also known as mummichog, are found in brackish water along the Atlantic coast of the United States (Figure 1). These fish stay close to home and are remarkably tolerant to changes in the environment, such as temperature, salinity, oxygen levels, and pH. This adaptability has even allowed some populations to not only survive, but thrive in polluted environments. Killifish living in polluted regions are documented to survive up to 8,000 times the lethal concentrations of pollution of killifish living in clean regions!
One population of pollution-tolerant killifish is located in the Elizabeth River, a tidal estuary in the Chesapeake Bay Region of Virginia. During the 19th and 20th centuries, humans industrialized the area, creating a hub for naval and shipping activities. However, this quickly led to the accumulation of heavy metals, pesticides, and persistent organic pollutants in the surrounding water and sediment. Of particular concern are the polycyclic aromatic hydrocarbons (PAHs) that entered the environment from creosote waste, a chemical derived from coal-tar used in wood preservation treatment (Figure 2). Exposure to PAHs can cause cancer, suppress the immune system, and result in respiratory and cardiovascular disease. Due to vast amounts of contamination throughout this region, it was declared a Superfund Site and added to the EPA’s National Priorities List in 1990.
Scientists set out to uncover how these killifish were tolerating such harsh conditions. They sequenced and compared whole genomes of killifish from polluted and non-polluted regions and found a unique set of mutations common among pollution-tolerant killifish and absent among fish in non-polluted regions. These mutations were acquired due to the genetic diversity present in killifish prior to the pollution exposure. When subsets of the population were exposed, these mutations were positively selected, meaning they provided individuals with the right tools to tolerate the harsh conditions (Figure 3). Therefore, the individuals possessing these mutations survive longer than those without, enabling them to pass their tolerance on to the next generation. This process is referred to as natural selection, a key concept in evolution that explains how these populations rapidly evolved to tolerate pollution.
Laboratory studies discovered the mutated genes in adapted killifish allow these populations to be resistant to PAH-induced acute toxicity, cardiac malformities, liver damage, and cancer. In contrast, non-adapted populations experience harmful effects when exposed to PAHs. Mechanistic studies demonstrate these genetic differences alter how adapted killifish metabolize, or biotransform, and remove harmful chemicals from their bodies. Additionally, these differences help protect against toxic outcomes of PAH exposure by increasing antioxidant production. Antioxidants help eliminate reactive oxygen species, harmful substances produced by cells when undergoing stressful conditions. By altering these critical mechanisms, adapted killifish can limit the overall toxicity induced by exposure to PAHs and other pollutants.
However, the ability to survive in these polluted environments is not entirely a good thing. While selective mutations provide short-term benefits to survival, they result in decreased genetic diversity. Additionally, these PAH-adapted fish experience reduced mitochondrial function, resulting in limited energy production. This makes it more difficult to swim, increases risk of predation, and impacts the ability to seek out an optimum environment. Down the road, the adapted populations may be less equipped to cope with additional environmental stressors, such as increases in global temperatures or hypoxia. Remember, the broad genetic diversity of killifish was a major factor allowing these populations to initially adapt to living in a polluted environment. Without this genetic diversity and bioenergetic capacity, these once-adaptable killifish may struggle to survive long-term in an ever fluctuating world. So next time you hear of evolution in the context of the past, remember it is also occurring at this very moment and, eventually, killifish may be an anecdote our grandchildren will learn about in their science classes.
